This proposal addresses the need to develop a rational, cost effective, and scientifically robust approach to the development of topical rectal microbicides from preclinical to exploratory studies in humans. The individual projects explore distinct, but overlapping areas of microbicide research that, cumulatively, will define the future of rectal microbicide development. The key premise that underpins the proposal is that receptive anal intercourse (RAI) is a common activity, playing a significant role in the transmission of HIV infection. Heterosexual RAI is common, with rates of 5-10% in US women and is considered to be the most common route of HIV transmission in men who have sex with men. Failure to acknowledge the role that RAI plays in the global AIDS pandemic is likely to limit the success of intervention strategies and compromise attempts to develop safe and effective vaginal microbicides. This MDP Project application is a U19, a collaborative effort with NIH and private sectors, to initiate coordinated, multidisciplinary research involving many of the world's experts in disciplines related to rectal microbicide development. We have focused our efforts on one class of microbicides, the reverse-transcriptase (RT) inhibitors (PMPA, UC-781, and TMC-120), two of which are in clinical trials as vaginal microbicide form. We have the support of three corporate sponsors (Biosyn, Inc., Gilead Sciences, and Tibotec-Virco) to initiate these efforts. The "preclinical" phase includes cell line, intestinal explant and macaque studies of microbicide safety and efficacy (Project by McGowan) which will progress microbicides into exploratory human trials designed to optimize microbicide safety evaluation, provide initial ex vivo/in vitro efficacy data, and information about distribution and bioavailability of RT microbicides (Project by Corner). The goal, over 5 years, will be to assess, what are the most cost effective and predictive assays for use in future microbicide development. Projects by Gorbach will target the role of RAI on mucosal function, the behavioral correlates of RAI, and acceptability studies of candidate formulations. Failure to adjust for these modifiers of safety and efficacy may result in (1) falsely attributing RAI toxicity to the microbicide and (2) failing to demonstrate efficacy because of simultaneous enhancement of HIV transmission due to RAI. The information derived from these studies will be critical for RT development, but will also provide a rational basis for the development of other classes of rectal microbicides.